Effects And Mechanism Of Polyphenols Extracted From Longan Seeds On The Digestibility Of Maize Starch

Longan seed is the main by-product of longan processing,which contains a lot of polyphenols and other active substances.This thesis used longan seed as raw material,after coarse extraction of polyphenols with 70%ethanol,purification conditions using four types of macroporous resins were optimized.The effects of ethanol(desorption liquid)concentration on the static adsorption and desorption of longan seed polyphenols were studied,and their structures were characterized using high performance liquid chromatography(HPLC)-4-pole-time of flight tandem mass spectrum.The impacts of longan seed polyphenols on the digestibility of maize starch were studied.The interaction of longan seed polyphenols with alpha-amylase,and with maize starch were discussed.The main results are as follows:(1)The results of the purification and identification of longan seed polyphenols showed that AB-8 was an ideal resin for purification of longan seed polyphenols,and its static adsorption rate and desorption rate were 51.36%and 96.4%,respectively.When the desorption solution was 70%ethanol and the p H was 4,the static desorption effect was the best.Thirteen main chemical constituents were identified in 70%ethanol eluent,among which eight chemical constituents have not been reported in longan polyphenols.(2)The addition of 10%longan seed polyphenols had an obvious inhibitory effect on the digestion of maize starch,and better inhibitory effect on starch digestion was achieved when more longan seed polyphenols were added.With the increase in digestion time,the content of free longan seed polyphenols increased and reached equilibrium at 150 min.(3)The addition of 10%and 20%longan seed polyphenols had no significant effect on the gelatinization onset,peak and conclusion temperatures of maize starch,however 20%longan seed polyphenols significantly reduced the gelatinization enthalpy of maize starch.FT-IR results showed that the addition of longan seed polyphenols did not change the original functional groups and chemical bonds of maize starch,indicating that longan seed polyphenols and maize starch interact through non-covalent bond.XRD results suggested that lonan seed polyphenols and maize starch formed a hydrophobic v-type inclusion compound.Similarly results were obtained using ITC,indicating that there was hydrophobic interaction between maize starch and longan seed polyphenols,and there might be other weak interactions.(4)Longan polyphenols showed significant inhibitory effect on alpha-amylase,with an semi-inhibitory constant IC₅₀ of 3.02 mg/m L,and the inhibitory type was competitive inhibition.The results showed that longan seed polyphenols could react with alpha-amylase spontaneously,change the secondary structure of alpha-amylase.They could form complex by hydrogen bond and van der Waals force,and cause the endogenous static fluorescence quenching of alpha-amylase.Therefore longan seed polyphenols could decrease the activity of alpha-amylase by reducing the binding sites of alpha-amylase to maize starch,and prevent the formation of active centers of alpha-amylase.